A4 is an internationally agreed upon standard, letter size is a mostly US thing. It's the whole SI units vs imperial units debate all over again. Makes sense to have the international standard as default rather than a country specific one.
> Letter or ANSI Letter is a paper size commonly used as home or office stationery in the United States, Canada, Chile, Colombia, Costa Rica, Mexico, Panama, the Dominican Republic and the Philippines. It measures 8.5 by 11 inches (215.9 by 279.4 mm). US Letter-size paper is a standard defined by the American National Standards Institute (ANSI, paper size A), in contrast to A4 paper used by most other countries, and adopted at varying dates, which is defined by the International Organization for Standardization, specifically in ISO 216.
LaTeX was invented in the U.S. (as was Unix, as was the transistor), so it makes sense for the default to be letter size.
It makes sense for there to be a system-wide setting to override the software default, so folks who use some other standard can set it once and never worry about it again.
> Letter or ANSI Letter is a paper size commonly used as home or office stationery in the United States, Canada, Chile, Colombia, Costa Rica, Mexico, Panama, the Dominican Republic and the Philippines. It measures 8.5 by 11 inches (215.9 by 279.4 mm). US Letter-size paper is a standard defined by the American National Standards Institute (ANSI, paper size A), in contrast to A4 paper used by most other countries, and adopted at varying dates, which is defined by the International Organization for Standardization, specifically in ISO 216.
https://en.wikipedia.org/wiki/Letter_%28paper_size%29